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One or more embryos? A parasitoidic dilemma....

The small (1.5 mm) parasitoid wasp female in the photo is inserting her ovipositor into the egg of a moth, which she uses as a host for her young. After she has laid her own egg within the host, the growing embryo divides many times to form a clone of genetically identical wasp larvae. While the host feeds, moves and grows, the parasitoid larvae inside its body also grow – by feeding on the host’s internal tissues. Eventually, the larvae kill their host, complete their development inside its dead body and emerge as adult wasps to start a new generation. In some species, thousands of wasps can develop from a single egg.

A Copidosoma koehleri female during egg-laying. Photo by Nitzan Cohen and Miriam Kishinevsky.

This type of development, called polyembryony, seems to be incredibly efficient. Surprisingly, it is not widespread in animals and is actually quite rare among parasitoids. Our paper explores why, by simulating evolutionary competitions between polyembryonic wasps and wasps that produce just one offspring per egg. We asked which simulated conditions cause the polyembryonic wasps to lose the competition and go extinct. The simulations predicted that this will happen when the wasps produce many eggs and when hosts are small in size. We found support for these predictions by comparing experimentally measured egg loads and host sizes between species of the two development types, within the parasitoid family Encyrtidae.

Oikos Journal

Oikos is a journal issued by the Nordic Ecological Society and is one of the leading peer-reviewed journals in ecology. Oikos publishes original and innovative research on all aspects of ecology, defined as organism-environment interactions.